Mobile penetration and the proliferation of mobile computing devices such as smart phones are driving mobile service providers to deploy more base stations in their networks. Small cells, which are miniature low-power cellular base stations, provide cellular coverage improvement as well as additional capacity. They provide a low-power signal much closer to mobile users than traditional macro networks, resulting in better voice quality, higher data performance and better battery life. By moving the base station closer to the user equipment, end users benefit from more reliable data connections and higher data throughput. They fit well in dense residential and business areas and also have the potential to help rural areas improve connectivity. Small cells types include femtocells, picocells, metrocells and microcells—generally increasing in size from femtocells (the smallest) to microcells (the largest).
Small cells enable operators to provide a higher quality mobile signal where it was never previously economical and/or possible to install conventional base stations and radio network equipment. Such places include indoor environments (e.g., residential apartments, malls, enterprises, public space hotspots, underground facilities, etc.) and many remote outdoor locations. Small cells also enable operators to meet the growing demand for mobile data, by extending the data capacity of the macro network at a fraction of the cost per gigabit (of macro base stations) mainly due to the following reasons: they have simpler design providing low-cost installation and automatic operation; they work with almost any internet connection for backhaul; and they make highly efficient use of valuable spectrum assets of the mobile operator.
Small cells offer mobile service providers a cost-effective alternative to macro-only deployments for meeting growing coverage and capacity demands. Plus, as small cells are added, they offload traffic from the macro network. This increases available network capacity without the deployment of new macro sites. Nevertheless, small cells require, in particular, an economical and, preferably, a readily available backhaul solution to serve the large number of small cells. They require access to the rest of the mobile infrastructure such as macro-cells, clock references and mobile network controllers. They depend on other parts of the infrastructure to provide mobile connectivity, clock synchronization information, backhaul and transport.
Leveraging the existing xDSL infrastructure for mobile backhaul conceivably can lead to lower access costs than traditional leased line services. The x stands for the first letter of the various Digital Subscriber Line (DSL) technologies, for example, ADSL and its variants (Asymmetric DSL), VDSL and its variants (Very high bit rate DSL), etc. It is important to note that in many cases the same infrastructure is also being used for Ethernet services to residential, corporate and enterprise customers, which means that it is suitable for Ethernet/IP interfaces and routing. xDSL solutions have always been mainly used for broadband services but have become relevant for small cell backhaul due to their low cost and pervasiveness.
FIG. 1 shows a schematic arrangement for time transfer to femtocell base stations over networks having asymmetric transmission rates. A radio controller 1 (having a grandmaster clock or timing reference) provides accurate time information to a plurality of slave devices over a packet network 2. A number of headends/aggregation points 3 are connected to the network. These headends 3 comprise either their own grandmaster clock or a boundary clock (BC) which is time and frequency synchronized to the grandmaster in the controller 1. One or more femtocells 4 are connected to the headends 3 over an existing broadband link which operates on a DSL protocol or other cable-based protocols. These femtocells 4 allow mobile devices 5 to access the network and receive voice and data in, for example, indoor environments.
However, the backhaul capacity varies depending on the xDSL technology used as well as the distance from the exchange.
The available data rates on DSL technologies depend on the copper length from the exchange (or DSL Access Multiplexer (DSLAM)). The data rates degrade over distance due to attenuation. The rates that can be delivered also depend on the quality and the gauge of the copper lines used. The bandwidth limitations of copper are being overcome by a technology known as DSL bonding. Multiple copper lines can be bonded to offer higher rates. This is based on adding copper capacity on demand and partitioning the traffic carried over the bonded links. Thus the aggregated traffic from a base station may be transported over multiple DSL links through the implementation of a bonding protocol like IMA (Inverse Multiplexing over ATM) or M2DSL (Multi-Megabit Digital Subscriber Line). Alternatively, bonding could be implemented in a way that is transparent to the DSLAM and based on ML-PPP (Multi-Link Point-To-Point Protocol). New technologies such as vectoring, which employs the coordination of line signals to reduce crosstalk levels (ITU G.993.5), can be used to improve performance.
Synchronization Issues
Synchronization plays a crucial role in mobile backhaul networks. Mobile wireless base stations (both Frequency Division Duplexing (FDD) and Time Division Duplexing (TDD) technologies) derive their carrier radio frequencies (RF) from a highly accurate reference clock. In case of FDD operation, there are two carrier frequencies, one for uplink transmission and one for downlink transmission. Small cells require the RF output to be accurate to within 100 parts per billion (ppb) for carrier-deployed picocells, and 250 ppb for home base stations. Macrocells have a narrower tolerance of 50 ppb, because it is assumed that the user equipment (UE) can be moving, creating a Doppler frequency shift between the UE and the macrocell. A frequency outside these limits may lead to dropped calls during handover between cells, or to the UE being unable to acquire the RF signal from the base station.
In case of TDD operation, there is only one single carrier frequency and uplink and downlink transmissions in the cell are always separated in time. To avoid severe interference between uplink and downlink transmissions despite the fact that the two links use the same frequency, the cells in a TDD network typically use the same uplink downlink configuration together with inter-cell synchronization to a common time reference to align the switch-points among all the cells. This is because downlink and uplink transmission is partitioned into different timeslots, which have to be co-ordinated between adjacent cells. This avoids interference between the two links as uplink and downlink transmissions do not occur at the same time. Small cells require a phase difference of less than 3 μs between base stations of adjacent cells. This is normally achieved by ensuring that the base stations are synchronized to within ±1.5 μs.
4G/LTE-Advanced, in particular, requires even stricter clock distribution accuracy to all base stations to ensure support for new features like Coordinated Multi-Point (COMP) operation, enhanced Inter-Cell Interference Coordination (eICIC), Carrier Aggregation, and Location Based Services (LBS). The accuracy required by each of these techniques has not yet been agreed by 3GPP.
Synchronization may be provided to small cells by distributing either a frequency or packet-based timing reference over the backhaul network. Alternatively, it can be provided by radio, or be built into the base station itself (using a chip-scale atomic oscillator, for example). IEEE 1588 PTP is now the industry accepted packet-based method/standard for distributing timing information from a master to enable the clocks of distributed systems to be synchronized with high precision (accuracies in the nanosecond levels). It is also designed for applications that cannot bear the cost of a GPS receiver plus antenna at each node, or for which GPS signals are inaccessible. Installing a GPS antenna on every cellular base station has consequences in terms of both capital expense and operational expense. In particular, it can add more effort and expenses in the initial installation process of the base station (additional antennas, wiring, receiver) as well as ongoing maintenance (technician time whenever the outdoor antenna requires maintenance, etc.).
Relying on GPS for clock synchronization is not always practical because an unobstructed sky-view to GPS satellites is required (a major problem for the emerging small cell technologies that are mainly targeting buildings, underground facilities, tunnels, closed spaces such as shopping malls, etc.) or its acceptance in certain regions of the world may be hindered by policies based on the reasoning that GPS is American owned technology and infrastructure. Telecom service providers around the world (outside North America) have recognized that and understand that there could be the possibility that one day the general availability and/or accuracy of the GPS service could be terminated and not made available for use outside America. Thus, relying on GPS has drawn strong geopolitical undertones making many countries in Europe and Asia reluctant or apprehensive to make their strategic telecommunications assets dependent on a foreign own asset—the GPS. To find a way out of this dependency, new GNSS (Global Navigation Satellite System) projects like the European Galileo project, the Russian GLONASS and the Chinese Beidou navigation system were initiated. However, up to date, the only fully operational GNSS system with full world coverage existing today is GPS. Another factor that could pose a problem to GPS use is the issue of GPS vulnerability, particularly, GPS jamming (where a GPS receiver could be easily jammed). This could easily be carried out nowadays using off-the-shelf consumer electronics GPS receivers.
The delivery of accurate synchronization to small cell base stations is one of the major challenges facing mobile network operators. One existing approach for bigger base stations like macrocells is to co-locate a GPS receiver and antenna at each base station. But this will not be economical for small cells networks, which most likely will see an exponential growth in the number of cell sites, each covering a relatively small slice of the area covered by a single macrocell base station. Moreover, small cell base stations are much smaller than macrocell base stations, and in many cases they will be unable to host a GPS receiver and antenna, either because there is no space or because there is no clear sky view to a GPS satellite, or both. Also, the issue is simply a matter of economics because purchasing, installing and maintaining a GPS receiver and antenna at every small cell base station will quickly become prohibitively expensive. Furthermore, small cells deployed in the indoor environment are unlikely to be able to acquire a GPS signal, due to the attenuation of the signal caused by the building in which they are housed, and will need to use a network-delivered reference. Hence, transferring timing via packet transport is a relevant synchronization technique that must coexist and interoperate with all others.
However, the challenge in clock transfer over packet networks is dealing with the packet delay variation (PDV) and communication path asymmetries (i.e., asymmetry in forward and reverse path delays) in the network, which can present considerable error components in the recovered clock. These error components have to be adequately mitigated in order to obtain accurate clock performance. The communication path asymmetries can be due to the following: physical link (wire/fiber) asymmetries; internal device signal path and processing delay asymmetries (chip, board, and board-to-board level delays); and load (or queuing) induced asymmetries (which arise when the queuing delays in the forward and reverse paths are different).
An additional source of communication path asymmetry is transmission rate asymmetry which occurs when the forward and reverse paths have different transmission rates resulting in a transmission delay asymmetry. The transmission delay (which is inversely proportional to the transmission rate) is the amount of time it takes to put data on the transmission medium. The higher the transmission rate, the shorter the transmission delay.
Time transfer using a protocol such as IEEE 1588 PTP and a well designed slave clock recovery mechanism can provide time synchronization in the sub-microsecond level and lower. However, this is done using the important assumption that the message transfer delay from master to slave is equal to that from slave to master. In real life, the communication paths are not perfectly symmetric mainly due to dissimilar forward and reverse physical link delays, internal device delays, and queuing delays. Even in cases where the physical link and internal device delays are known and properly compensated for during clock synchronization, queuing delays which are variable can still exist when timing messages go through the packet network and queued for forwarding.
A Transparent Clock (TC) is a fundamental element of an IEEE 1588 synchronization network. It is a network node with special IEEE 1588 functionality and its purpose is to capture the residence time of PTP message as they traverse the network node. This information can then be used by the slave to mitigate the adverse effect of PDV on the transfer of frequency and time without the protocol complexity inherit to terminating a PTP session as would be in the case of using a Boundary Clock (BC). A TC updates the correction field in PTP packets as they traverse the TC. The updates account for the cumulative delay introduced by TCs between master and slave. This cumulative delay can be used to compensate for the synchronization error a slave clock experiences as a result of PDV effects when locking to the GM or BC. BCs provide a means to distribute synchronization information in larger networks by building a clock hierarchy. The BC relieves the GM from handling a large number of ordinary clocks by segmenting downsteam networks into areas.
In xDSL and GPON, for example, the inherent transmission rate asymmetry is additional source of delay asymmetry that needs to be properly addressed during clock synchronization.
Overview of IEEE 1588v2 PTP
The IEEE 1588v2 PTP defines a packet-based synchronization protocol for communicating frequency, phase and time-of-day information from a master to one or more slaves with sub-microsecond accuracy. PTP relies on the use of accurately timestamped packets (at nanosecond level granularity) sent from a master clock to one or more slave clocks to allow them to (frequency or time) synchronize to the master clock. Synchronization information is distributed hierarchically, with a GrandMaster clock at the root of the hierarchy. The GrandMaster provides the time reference for one or more slave devices. These slave devices can, in turn, act as master devices for further hierarchical layers of slave devices. PTP provides a mechanism (i.e., Best Master Clock Algorithm) for slave clocks to select the best master clock in their respective synchronization domain. The selection is performed according to the PTP attributes of the GrandMaster (e.g. PTP priority, clock class).
The typical PTP message exchange process (i.e., the PTP Delay Request/Delay Response flow) between a master 10 and a slave 30 is performed as follows and illustrated in FIG. 2. IEEE 1588 PTP allows for two different types of timestamping methods, either one-step or two-step. One-step clocks update time information within event messages (Sync and Delay-Req) on-the-fly, while two-step clocks convey the precise timestamps of packets in general messages (Follow_Up and Delay-Resp). A Sync message is transmitted by a master to its slaves and either contains the exact time of its transmission or is followed by a Follow_Up message containing this time. In a two-step ordinary or boundary clock, the Follow_Up message communicates the value of the departure timestamp for a particular Sync message.
FIG. 2 illustrates the basic pattern of synchronization message exchanges for the two-step clocks. The master 10 sends a Sync message to the slave 30 over the network 2 and notes the time T1 at which it was sent according to the clock in the master 10, which is synchronized with a clock reference 11 such as the GPS. The slave 30 receives the Sync message and notes the time of reception T2. The master conveys to the slave the timestamp T1 by one of two ways: 1) Embedding the timestamp T1 in the Sync message (one-step clock)—this requires some sort of hardware processing (i.e., hardware timestamping) for highest accuracy and precision; or 2) Embedding the timestamp 1; a Follow_Up message. Next, the slave 30 sends a Delay_Req message to the master and notes the time T3 at which it was sent according to the slave clock 31. The master 10 receives the Delay_Req message and notes the time of reception J. The master 10 conveys to the slave 30 the timestamp T4 by embedding it in a Delay_Resp message.
At the end of this PTP message exchange, the slave 30 possesses all four timestamps {T1, T2, T3, T4}. These timestamps may be used to compute the offset of the slave's clock 31 with respect to the master and the communication delay of messages between the two clocks. The computation of offset and propagation time often assumes that the master-to-slave and slave-to-master propagation times are equal, i.e. that the communication paths are symmetrical. Clock frequencies change over time, so periodic message exchanges are required. Because these clock variations change slowly, the period between message exchanges is typically on the order of milliseconds to seconds.
The present invention aims to provide methods and systems which improve the accuracy of time synchronization over links which have inherent asymmetries in the communication paths, such as existing broadband infrastructure.
This may allow the adoption of highly accurate time synchronization over such communication paths, thereby permitting wide deployment of small cell base stations for mobile telecommunications.
Small cells are a critical component of 4G/LTE ultra-broadband deployments as they enable operators to boost coverage and capacity in busy areas such as shopping malls, sports stadiums and high-traffic roads. By bringing the base station closer to the end user, small cells assure exceptional mobile broadband including enriched multimedia services wherever consumers are located. Small cells require, in particular, an economical and, preferably, a readily available backhaul solution to serve the large number of small cells. They require access to the rest of the mobile infrastructure such as macro-cells, clock references and mobile network controllers. Leveraging the existing broadband (e.g., xDSL, GPON) infrastructure for small cells backhaul conceivably can lead to lower access costs than traditional leased line services. For time synchronization, the quality of the time distribution depends heavily on the level of packet delay variation (PDV) and communication path asymmetries between the clock source (master) and the receiver (slave). An additional source of communication path asymmetry (that exists in technologies such as xDSL and GPON) is transmission rate asymmetry which occurs when the forward and reverse paths have different transmission rates resulting in a transmission delay asymmetry. Transmission delay asymmetry further complicates the problem of clock recovery at the end-user (slave).
Therefore, an object of the present invention is to provide systems and methods for accurate time transfer over links with asymmetric transmission rates (e.g., xDSL, GPON).